The Embryonic Stem Cell (ES) core is designed to improve the efficiency of the Program Project Laboratories for all phases of ES cell work. This core will facilitate the production of targeted ES cell lines, will assist laboratories in their production of chimeric animals and mice with germline mutations in targeted alleles, and will explore the use of embryonic stem cells as models of hematopoietic differentiation. The objectives of this core are listed below: 1. The core will serve as a repository for all characterized ES cell lines. 2. The core will produce and distribute mouse embryo fibroblasts (MEFs) from normal and transgenic neo-resistant mice. 3. The ES core is responsible for injecting targeted ES cell lines into blastocysts for the production of chimeric mice. 4. The ES core and transgenic cores are responsible for the breeding programs of chimeric mice to establish germline transmission of target alle s. 5. The ES core will create and characterize new embryonic stem cell lines. These lines are being produced for use in gene targeting experiments, and will improve the efficiency of the core by reducing the number of blastocyst injections required for each targeted ES line.